title: Lifespan trajectory of myelin integrity and maximum motor speed.
abstract: OBJECTIVE
  Myelination of the human brain results in roughly quadratic trajectories of myelin content and integrity, reaching a maximum in mid-life and then declining in older age. This trajectory is most evident in vulnerable later myelinating association regions such as frontal lobes and may be the biological substrate for similar trajectories of cognitive processing speed. Speed of movement, such as maximal finger tapping speed (FTS), requires high-frequency action potential (AP) bursts and is associated with myelin integrity. We tested the hypothesis that the age-related trajectory of FTS is related to brain myelin integrity.
  
  
  METHODS
  A sensitive in vivo MRI biomarker of myelin integrity (calculated transverse relaxation rates (R(2))) of frontal lobe white matter (FLwm) was measured in a sample of very healthy males (N=72) between 23 and 80 years of age. To assess specificity, R(2) of a contrasting early-myelinating region (splenium of the corpus callosum) was also measured.
  
  
  RESULTS
  FLwm R(2) and FTS measures were significantly correlated (r=.45, p<.0001) with no association noted in the early-myelinating region (splenium). Both FLwm R(2) and FTS had significantly quadratic lifespan trajectories that were virtually indistinguishable and both reached a peak at 39 years of age and declined with an accelerating trajectory thereafter.
  
  
  CONCLUSIONS
  The results suggest that in this very healthy male sample, maximum motor speed requiring high-frequency AP burst may depend on brain myelin integrity. To the extent that the FLwm changes assessed by R(2) contribute to an age-related reduction in AP burst frequency, it is possible that other brain functions dependent on AP bursts may also be affected. Non-invasive measures of myelin integrity together with testing of basic measures of processing speed may aid in developing and targeting anti-aging treatments to mitigate age-related functional declines.
authors.affiliation: Department of Psychiatry and Biobehavioral Sciences, The David Geffen School of Medicine at UCLA, Los Angeles, CA 90024, USA. gbar@ucla.edu
authors.forename: George
authors.initials: G
authors.lastname: Bartzokis
authors.forename: Po H
authors.initials: PH
authors.lastname: Lu
authors.forename: Kathleen
authors.initials: K
authors.lastname: Tingus
authors.forename: Mario F
authors.initials: MF
authors.lastname: Mendez
authors.forename: Aurore
authors.initials: A
authors.lastname: Richard
authors.forename: Douglas G
authors.initials: DG
authors.lastname: Peters
authors.forename: Bolanle
authors.initials: B
authors.lastname: Oluwadara
authors.forename: Katherine A
authors.initials: KA
authors.lastname: Barrall
authors.forename: J Paul
authors.initials: JP
authors.lastname: Finn
authors.forename: Pablo
authors.initials: P
authors.lastname: Villablanca
authors.forename: Paul M
authors.initials: PM
authors.lastname: Thompson
authors.forename: Jim
authors.initials: J
authors.lastname: Mintz
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000375:Aging / Q000473:pathology / Q000502:physiology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D009044:Motor Cortex / Q000502:physiology* / Q000648:ultrastructure; D009068:Movement / Q000502:physiology*; D009186:Myelin Sheath / Q000502:physiology* / Q000648:ultrastructure*; D010807:Physical Endurance / Q000502:physiology*; D005082:Physical Exertion / Q000502:physiology*; D055815:Young Adult